Two molecular forms of gastri, G-17 and G-34, are potent stimulants of gastric acid secretion in man. G-34 is more abundant in human serum but G-17 is more potent on a molar basis. Both are released by a protein meal. The goal of the present investigation is to quantitate changes in circulating G-17 and G-34 after a protein meal and to measure acid secretion by the method of in vivo intragastric titration. Solutions of pure G-17 and G-34 will be given intravenously to the same subjects at rates sufficient to produce similar increments in circulating hormone concentration achieved after the meal. We will try to determine whether acid secretion produced by these peptides is as great as acid secretion produced by the meal in order to define more clearly the role of gastrin as a physiological stimulant of acid secretion. Another project involves bioassay of plasma extracts from patients with basal acid hypersecretion and control subjects in an effort to demonstrate the presence of circulating stimulants of acid secretion distinct from gastrin.